feat(driver): add FocusedWindowChecker capability

internal/driver/driver.go

@@ -47,6 +47,18 @@ type ForegroundChecker interface {
 	ForegroundApp(ctx context.Context) (string, error)
 }
 
+// FocusedWindowChecker is the optional capability for reporting which app owns
+// the focused (on-screen) window. The startup gate prefers it over
+// ForegroundChecker: the resumed-activity signal flips to a freshly launched
+// app before its first frame draws, so observing on it alone can capture the
+// previous app's screen. The focused window only names the app once its window
+// is actually up.
+type FocusedWindowChecker interface {
+	// FocusedWindowApp returns the package owning the focused window, or ""
+	// when no window is focused yet (e.g. mid-launch transition).
+	FocusedWindowApp(ctx context.Context) (string, error)
+}
+
 type LogEntry struct {
 	UnixMillis int64
 	Level      string

internal/driver/sidecar/client.go

@@ -33,6 +33,16 @@ func (c *Client) ForegroundApp(ctx context.Context) (string, error) {
 	return android.ForegroundPackage(ctx)
 }
 
+// FocusedWindowApp reports the package owning the focused window. Only Android
+// is supported (via adb); other platforms return "" so the startup gate falls
+// back to the foreground-app signal.
+func (c *Client) FocusedWindowApp(ctx context.Context) (string, error) {
+	if c.platform != "android" {
+		return "", nil
+	}
+	return android.FocusedWindowPackage(ctx)
+}
+
 // Dial connects to the sidecar gRPC server at the given address.
 // Address must be a host:port pair, typically "127.0.0.1:<sidecar-port>".
 func Dial(address string) (*Client, error) {